This close-up shows how the CalTracs bar link is connected to the front spring eye mountin
Jeff Smith: At first, the information sounded a bit contradictory, so I called my friend Doug Norrdin at Global West Suspension Components, who brought up a few excellent points. The first thing that didn't make sense was your attempt to use bolt-on traction bars, yet the car still axle-tramped. As you are painfully aware, this is a violent condition that (with leaf spring cars) is caused when the front half of the spring section wraps up, making it bend into an S shape. When the spring is sufficiently distorted, it binds, and because of this induced infinite spring rate, it bounces the tire off the pavement. The process repeats itself very rapidly and if continued can cause massive damage to the trans, driveshaft, and rearend-as you have unhappily discovered. A simple way to control spring wrapup is to use a bolt-on traction bar. A typical traction bar employs a polyurethane snubber that prevents the spring (in theory) from bending. One thing that may have happened in your case is an excessive air gap between the snubber and the spring with the car at ride height. Very few bolt-on traction bars are designed to make contact in the ideal location, which is directly under the front spring eye. Most bars are made too short, placing the snubber somewhere behind the front spring eye. If this is combined with a large air gap, the spring can still wrap up before the snubber makes contact. In extreme cases, it can still allow wheelhop. Norrdin also mentioned that the KYB shocks are not a good choice for this application. These shocks do not have the sufficient extension (more often called rebound) valving that is necessary to control wheelhop. It's possible that it was this combination of soft rebound control and excessive air gap that caused the problem.
Next, the reason you rarely see those Shelby under-ride bars on most cars these days is because back in the mid-'60s when they were developed, the hot ticket for road racing was to slide the car through the corner, hanging the back out in the turn in what is now called drifting. This was accepted practice back in the day because the tires were rock hard. The Shelby bars contributed to that technique because the front pickup point for the bar did not travel in the same arc as the front spring eye. This caused the rear suspension to bind, putting the car into a slide. This bind will also cause wheelhop.
Also aggravating the above situations are the short-sidewall 275/40R17 tires. As the sidewall becomes shorter, it increases in stiffness, the exact opposite of a typical drag tire that employs a very pliable sidewall to absorb shock loads. All this contributes, Adam, to a perfect storm of solutions that didn't work. We have a better idea. John Calvert is a Ford drag racer who developed a new-design traction bar many years ago that he calls the CalTracs. While at first glance it looks like a Shelby bar, it is far different because the front pickup point and the front spring eye swing through the same arc, so there's no bind. A stock leaf spring rear suspension pushes power through the front section of the spring during acceleration, which is why the spring bends. A stiffer front spring section will help resist this tendency to wrap up, which is why just adding a section of leaf spring to the front half of the spring will reduce wheelhop-you could try this first if you like. Adding the CalTracs bar below the spring changes the way power is transferred to the ground. The torque is now pushed through the CalTracs bar, which does not bend. Another plus is that this allows the suspension tuner to lower the rear spring rate and tune the rear suspension to allow the car to negotiate corners without excessive oversteer. This is the negative to adding an additional leaf to the front half of the existing spring because it also increases the rear spring rate, which will tend to cause the car to oversteer during cornering. The CalTracs bar is probably the best solution, along with a tunable shock absorber, such as a Koni, that allows separate rebound adjustment.
Global West Suspension Components
San Bernardino, CA
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Car Craft Mag
831 S. Douglas St.
El Segundo, CA 90245
We spotted this Hennessey Camaro at a Sunday morning car show. This is the HPE700 with a Corvette ZR1 supercharged LS9 engine. Tweaked by Hennessey, these things make 725 hp, 741 lb-ft, and will accelerate to 60 in a scant 3.5 seconds. Keep your foot in it, and the quarter-mile will pass you by in 11.3 seconds.